The group III nitride compound semiconductor can realize high-efficiency light emission because of its direct-transition type bandgap energy corresponding to a region from visible to ultraviolet and therefore, is produced as an LED or LD. It is therefore possible to produce an electronic device having properties unachievable by conventional group III-V compound semiconductors.
A single crystal wafer of a group III-V compound semiconductor is not yet commercially available, and a method of growing a crystal of a group III-V compound semiconductor on a single crystal wafer of a different material is generally employed. A large lattice mismatch is present between such a dissimilar substrate and a group III nitride compound semiconductor crystal epitaxially grown thereon. For example, there is a lattice mismatch of 16% between sapphire (Al2O3) and gallium nitride (GaN) and 6% between SiC and gallium nitride. In general, the presence of such a large lattice mismatch makes it difficult to epitaxially grow a crystal directly on the substrate and even when a crystal is grown, a crystal with good crystallinity cannot be obtained. Accordingly, in the case of epitaxially growing a group III nitride compound semiconductor crystal on a sapphire single crystal substrate or an SiC single crystal substrate by the metal-organic chemical vapor deposition (MOCVD) method, there is generally employed a method where, as disclosed in Japanese Patent No. 3,026,087 and Japanese Unexamined Patent Publication No. 4-297023, a layer called a low-temperature buffer layer composed of aluminum nitride (AlN) or AlGaN is first deposited on the substrate and the group III nitride compound semiconductor crystal is then epitaxially grown thereon at a high temperature.
Furthermore, a technique using a columnar crystal texture layer as the buffer layer is described in Unexamined Patent Publication No. 2003-243302 and Journal of Crystal Growth, Vol. 115, pp. 628-633 (1991). The technique disclosed in these publications uses the MOCVD method for the growth, similarly to conventional techniques above. The MOCVD method is suitable to form a high-quality crystal film at a high growth rate, but for the formation with good uniformity of a film having a structure such as columnar crystal, a growth method using a plasmatized metal raw material, such as sputtering, is suited. The above publications are silent on the columnar crystal density.
On the other hand, as regards the technique of growing a buffer layer by a method other than MOCVD, several reports have been made. For example, Japanese Examined Patent Publication No. 5-86646 describes a technique of growing a buffer layer by high-frequency sputtering and growing thereon a crystal having the same composition by MOCVD. However, Japanese Patent Nos. 3,440,873 and 3,700,492 reveal that a good crystal cannot be stably obtained only by the technique described in Japanese Examined Patent Publication No. 5-86646. For stably obtaining a good crystal, it is important, as described in Japanese Patent No. 3,440873, to anneal the grown buffer layer in a mixed gas composed of ammonia and hydrogen, or as described in Japanese Patent No. 3,700,492, to film-form the buffer layer at a temperature of 400° C. or more by DC sputtering.
However, these patent publications are silent on what crystallinity is preferred for the layer that is film-formed on the substrate. In practice, according to the results in intensive experiments performed by the present inventors, a group III nitride compound semiconductor crystal cannot be stably obtained as a good crystal only under the conditions described in the patent publications above.
As for the substrate, Japanese Patent Nos. 3,440,873 and 3,700,492 recite sapphire, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, a group III nitride-based compound semiconductor single crystal and the like, and it is stated that above all, an a-plane sapphire substrate is most matched.